You can visualize your standard 3 dimensions with one dimension as left-right, one as up-down, and the other as forward-backward. Displaying 3 dimensions on a page is time-consuming, so they use "into the page" for forward and "out of the page" for backward.

I can't really say why something happens, but an electric field that changes over time can induce a magnetic field. In a current, you have a net movement of charges, so the electric field of a charge at some point is changing due to that charge moving farther away.

also can you teachme how to do this question,
an electron is traveling north through a magnetic field that is vertically upward. in what direction will the magnetic force be?
for this question , if i use left hand rule, how is it possible to move the thumb and and finger both north(upward?)
ty!

Staff: Mentor

also can you teachme how to do this question,
an electron is traveling north through a magnetic field that is vertically upward. in what direction will the magnetic force be?
for this question , if i use left hand rule, how is it possible to move the thumb and and finger both north(upward?)
ty!

We use the right hand rule by convention, not left.

The correct equation to use is the Lorentz force equation. Can you show us that equation? Are you familiar with how the vector cross product in that equation works?

Staff: Mentor

but my teacher said we use left hand rule, in Calgary
i am not sure, in the textbook, it said use left hand for electron, and right hand for positive

In my opinion, that is a bad way to teach the Lorentz Force equation. When there is no electric field (just magnetic field as in your problem statement), the Lorentz Force on a moving charge is given by:

F = qv X B

Where F is the resultant vector force, q is the charge, v is the vector velocity, and B is the vector magnetic field.

q has a + sign if it is positive charge, and a - sign if it has negative charge. That changes the direction of the first vector in the cross product, which is why you use the RHR always, and get the correct answer for both negative and positive charges.

Staff: Mentor

Which is what you are doing with the Lorentz force equation. qv crossed into B gives you the force F. If the sign on the charge is negative, that turns the direction of the qv vector around, so you still use the RHR but with the qv vector pointing the opposite way.